APPENDIX 253 acid into the acid chloride. Bromine in presence of phosphorus forms in the same way, first, the acid bromide, and in the second stage of the reaction, the bromine substitution product. The bromine in all cases attaches itself to the a-carbon (z>., next the carboxyl). Where no free hydrogen exists in this position, as in trimethylacetic acid, no substitution occurs. Iodine can be introduced by the action of KI on the bromine derivative, CILjIJr.COOIl -I- KI = C1U.COOH + KBr. Monohalogen derivatives may also be obtained from the unsaturated acids by the action of the hydracids (HC1, HBr, HI). In this case the halogen attaches itself to the carbon farthest from the carboxyl. Thus acrylic acid gives with HBr the /3-bromopropionic acid, rilo-.CII.CO.OII ! HI5r~CII.jlir.CILj.COOH. The action of the hydracids, PClr, andPI3r5, on the hydroxy- acids also yields the halogen derivatives, riI:1.CI!(OiI).CO()II -I HBr. = CILj.CIIBr.COOH + HaO. au<:ii(<>ii).coon >i 2pci, = cn,.cnci.coci + 2?6ci, -I-2I1C1. In the latter case the acid chloride must be subsequently decomposed by water to obtain the acid. The increase in the number of halogen atoms in the acid raises the boiling point as well as the strength of the acid as determined by its dissociation constant K. 15.1'. K. Act-tic ucid....... 118° '0018 Monochlonvct'lie acid . . . 185° "155 Diclilorucctic ucid..... 190° 5*14 Trichloracctic ucid .... 195° 121 Some of the transformations of monohalogen acids are illustrated by the following equations : CII.,n.<'<><)! I t II.jO (.'ILjOH.COOII -i I1CL CILCl.COOIl I KCN ~ CHyCN.COOII -I K.C1 CIIoCl.C'OOII ! 2NH:, •- CIIyNIUCOOlI I N1I4CL £C'U.,Iir.C(.'K)II I Ag.. -=--• | " + 2AgBr CIIo.COOH T KOII - CII2":CI1.COOH + KI + H2O.